1. Field of the Invention
The present invention relates to an ultrasonic speaker system that reproduces signal sound in an audiofrequency band, using non-linearity of a medium (e.g., air) to an ultrasonic wave.
2. Description of the Related Art
An ultrasonic speaker system, having very strong directivity as compared with an ordinary speaker, is used for the purpose of transmission of speech information only to a specific place, or the like. For example, in an exhibition hall such as an art museum and the like, an ultrasonic speaker is installed so that when people approach a certain work, only a person within a limited range near the work can hear an explanation about the work.
FIG. 2 is a diagram showing a configuration of a conventional ultrasonic speaker system. As shown in FIG. 2, the conventional ultrasonic speaker system has modulation block 102, amplifier 103, and ultrasonic actuator 104. A speech signal is outputted from speech signal source 101 and the speech signal is inputted to modulation block 102. In modulation block 102, a modulated signal is generated based on the speech signal, and the modulated signal is inputted to amplifier 103. Amplifier 103 amplifies the modulated signal outputted from modulation block 102, and the amplified modulated signal is inputted from amplifier 103 to ultrasonic actuator 104. Ultrasonic actuator 104 converts the modulated signal to an acoustic wave of a finite amplitude level and radiates the acoustic wave of the finite amplitude level to a medium.
As one example of the modulated signal generated in modulation block 102, an AM modulated signal that modulates an amplitude of an ultrasonic wave (carrier wave) by a speech signal is considered.
As related-art document information regarding the invention of the present application, for example, Patent Document 1 has been known. (Patent Document 1: Unexamined Japanese Patent Publication No. S60-75199)